In recent years, a low-power-consumption liquid crystal display device (LCD) that is thin, lightweight, and capable of being driven by a lower voltage has been widely used. In such a liquid crystal display device, a screen size has been increasing year after year and a display of moving images just like TV images has been required. To meet such requirements, wiring should be configured with a low-resistive and high-conductive material. In response, in recent years, the wring for this practice is configured to be lower in resistance compared to an aluminum alloy, that is, configured by using a copper (Cu) which is higher in conductivity.
Along with such an increase in the screen size of the liquid crystal display device, a gate wiring material has been changed from molybdenum alloy to aluminum alloy or aluminum clad. However, aluminum (Al) has problems of hillock and electromigration.
For example, as disclosed in Patent Document 1, Japanese published unexamined patent application No. 2000-199054, a wiring material consisting of Al—Nd alloy has been proposed, and anodized Al, Mo-alloy-cladded Al, or double-layered Al has been used. The Al—Nd alloy has a resistivity of 5.1 μΩ·cm while pure Al has a resistivity of 2.5 μΩ·cm.
Therefore, in order to use pure Al in practice, wiring having a three-layered structure of Ti/Al/Ti or Mo/Al/Mo is used as measures for the hillock and electromigration. However, this causes an increase in film formation steps, which remains as a problem.
On the other hand, copper has drawn attention as a material having resistivity lower than those of the above-described gate electrode materials. However, copper has problems of not only poor adhesiveness to glass used as a TFT substrate in an LCD, but also of being easily oxidized when an insulation layer is formed
For this reason, a technique using alloyed copper wiring has recently been attempted in a TFT-LCD in order to solve such problems. This technique is intended to make an alloying element form reaction products with a substrate to thereby ensure adhesiveness to the substrate as well as to make an additive element form oxide on the Cu surface to thereby make the oxide act as an oxidation resistant film for Cu.
However, with this proposed technique, the intended characteristics have not been adequately developed. That is, the electrical resistance of Cu becomes higher because the alloying element remains in Cu, and therefore advantages over wiring materials using Al or Al alloy have not been able to be exhibited.
Further, as described in Patent Document 2, Japanese published unexamined patent application No. 2004-163901, in order to use the copper wiring for a TFT-LCD, a technique to ensure adhesiveness and barrier characteristics to a substrate by the formation of a Mo alloy film between Cu and the substrate has been considered.
However, this technique has problems of an increase in process steps, i.e., a step of forming the Mo alloy film, as well as of an increase in wiring effective resistance. Further, a Cu single layer is used for source and drain electrodes, however, there remains a problem in its stability.
Also in Patent Document 3, Japanese published unexamined patent application No. 2004-139057, a technique in which a refractory nitride such as TaN, TiN, or WN is formed around Cu is proposed to solve such problems regarding the Cu wiring. However, compared with the conventional wiring materials, this technique has disadvantages of requiring another material to form a barrier layer and additional process steps, and of increasing the effective resistance of the wiring because a high-resistive barrier layer is thickly formed.
Further, Patent Document 4, Japanese published unexamined patent application No. 2005-166757 discloses a method for improving adhesiveness and oxidation resistance by the addition of one or more elements selected from Mg, Ti, or Cr to TFT-LCD wiring. However, this technique has a problem in that the additive element remains in the wiring, causing an increase in wiring resistance. Also, the additive element reduces oxide in a substrate and the reduced element is diffused in the wiring, causing a problem of an increase in the wiring resistance.
Patent Document 5, Japanese published unexamined patent application No. 2002-69550 discloses a method for improving oxidation resistance by the addition of 0.3 to 10 wt. % of Ag to Cu. However, this technique has problems in that adhesiveness to a glass substrate is not improved and oxidation resistance sufficient to resist liquid crystal processing cannot be obtained.
Patent Document 6, Japanese published unexamined patent application No. 2005-158887 proposes copper alloy prepared by the addition of 0.5 to 5 wt. % of at least one element of Ti, Mo, Ni, Al, or Ag to Cu to improve adhesiveness. However, there is a problem of an increase in electrical resistance of the wiring due to the additive element.
Patent Document 7, Japanese published unexamined patent application No. 2004-91907, proposes a method for suppressing oxidation by the addition of 0.1 to 3.0 wt. % of Mo to Cu followed by the segregation of the added Mo at grain boundaries. However, although this technique is capable of improving oxidation resistance of Cu, it has a problem of increasing wiring resistance.
In Patent Document 8, WO2006-025347, copper alloy prepared by the addition of an appropriate additive element to Cu is used, wherein the additive element forms an oxide film to become a passivation film, which suppresses oxidation of Cu, and the passivation film is also formed at an interface with an adjacent insulation layer, which suppresses interdiffusion. Consequently, copper wiring having high conductivity and superior adhesiveness to a substrate is provided. Further, a liquid crystal display device using such copper wiring is provided. In addition, it is suggested that one of such additive elements preferably be Mn. However, this technique is not sufficient for realizing characteristics of a wiring structure used in a liquid crystal display device and a electrode structure of TFT.
In Patent Document 9, Japanese granted patent publication No. 3302894, a TFT structure used in a TFT-LCD is proposed and a TFT structure, wherein a gate electrode coated with an oxide film when a Cu alloy is applied to the gate electrode, is specifically presented. In the document, it is presented that when Cu is a first metal, the second metal is at least one type chosen from Ti, Zr, Hf, Ta, Nb, Si, B, La, Nd, Sm, Eu, Gd, Dy, Y, Yb, Ce, Mg, Th, and Cr. However, the additive element of this second metal is different from that of the current invention.
A source electrode or a drain electrode structure in a TFT electrode is not mentioned in any of these documents. However, high adhesiveness to a semiconductor layer or a pixel electrode, resistance to a use environment, and stability as an electrical contact of the source electrode or drain electrode portion are required to the source electrode or the drain structure, and they are important components for a liquid crystal display device.
As described above, in these conventional techniques, an attempt to ensure adhesiveness to a semiconductor layer or a pixel electrode and oxidation resistance by the addition of an alloying additive element to Cu has been made, however, none of them has achieved a satisfactory result. Also, a satisfactory result has not been achieved regarding high adhesiveness to a semiconductor and a pixel electrode, resistance to a use environment, or stability as an electrical contact of source electrode or drain electrode which are required to the source electrode or the drain electrode structure in a TFT electrode.
Especially, although Patent Document 8, WO2006-025347, suggests a liquid crystal display device using such copper wiring, the suggested technique alone is not able to provide a satisfactory configuration to realize a gate wiring structure to be used for the liquid crystal display device. Also, in the Patent Document 9, the oxide film for coating the gate electrode is specified as the oxide film mainly comprising the second metal formed by thermally treating in an oxygen ambient environment, however, it is not only unexplained but also not even indicated in the Patent Document 9 that the thermal treatment reacting Cu alloy and Si oxide film in contact with the Cu alloy, thereby forming oxide film of gate electrode, which ensures adhesiveness with a semiconductor layer or a pixel electrode.
That is, all the difficulties in forming a Cu alloy film with less process steps, lowering the effective resistance of the wiring, and improving adhesiveness to a semiconductor layer or a pixel electrode must be solved, however, with the above conventional techniques, these difficulties cannot be solved, and product manufacturing is practically difficult.
The present invention has been made in consideration of such situations, and it is therefore one object of the present invention to form an oxide film having high adhesiveness to a semiconductor layer or a pixel electrode to thereby prevent oxidation of a wiring material including a source electrode or a drain electrode, and also to provide a liquid crystal display device consisting of a TFT structure having a source electrode or a drain electrode held between the semiconductor layer such as amorphous silicon, and a passivation layer, and a manufacturing method therefor. Further, another objective of the present invention is to form the wiring, electrode, or terminal electrode (especially the source electrode or the drain electrode) applicable to practical manufacturing processing while simultaneously accomplishing all the above objects.